1. Field of the Invention
The present invention relates to a device and method for measuring the orientation of the fractures or drains in a geological formation.
It applies to the field of fossile energy production and more particularly to the stimulation of reservoirs and relates both to vertical wells and to deflected wells.
2. Description of the Prior Art
Hydraulic formation fracturing consists in cracking the productive rock by increasing a fluid pressure in the well and maintaining the crack thus created open. It develops along a plane whose orientation depends on the forces exerted on the reservoir:
the main vertical force due to the weight of the sediments (.sigma..sub.1),
the main horizontal stresses which depend in particular on the tectonics of the side (.sigma..sub.2 and .sigma..sub.3).
The fracture plane develops perpendicularly to the lowest of these three stresses the fracture will in general be horizontal at a small depth (less than 600 m), the vertical stress being smaller than the two horizontal stresses, and vertical for greater depths, fracturing being perpendicular to the smallest of the two horizontal stresses.
Hydraulic fracturing is sometimes used for connecting two wells at the level of a geological formation, for example for carrying out the underground gasification of a cold air whose permeability is too low to provide, between the two wells, the gas flow rate required for maintaining back combustion.
It is also used for providing the connection between two wells in the case of high enthalpy geothermy, or for seeking better scavenging of an oil deposit by forming a drain which distributes the injection of water charged with chemical additives.
For all these methods, knowledge of the direction taken by the fracture created is essential. If such knowledge is indispensable when it is a question of using the fracture for connecting the two wells together, it is not less important for a simple stimulation where only an improvement in the productivity of the well is sought; in fact, if the fracture is directed towards the oil-water limit, it will cause premature submersion of the well which will cause its closure, instead of the expected increase in oil production.
It is known to seek the direction of a fracture by observing the wall of a well through an oriented television camera, or by using the technique of printing packer. A sealing member or packer equipped with a deformable membrane is lowered and anchored in the layer before and after fracturing. The fracture is visible on the membrane of the packer which has an orientation detection device.
These methods can only apply to uncased wells and imposes bringing the well to a long halt for putting in the apparatus and for their withdrowal.
It is also known to determine the fracture direction by an acoustic detection of the advance of the fracture, this detection can be remotely achieved when the well is not outfit and preferably uncased at less than 100 m from the fractured well.
Geophones or accelerometers applied against the wall detect noises related to the fracturing. However, the availability of such a listening well is fairly uncertain and, in addition, with the present interpretation methods, a direction even approximative of the fracture cannot be derived from the numerous noises recorded, for the time being.